The present invention relates to a continuous flow web coating method and apparatus, in general, and to such method and apparatus for controlling vibration-produced variations in web coating thickness where the coating is applied by continuous flow web coating apparatus, in particular.
There are many well-known methods of machine coating surfaces. These methods include spray coating, coating by transfer of the liquid directly to the surface to be coated by means of an applicator such as a brush or a roller, dip coating and methods wherein the surface to be coated is moved past a slot-like orifice through which the coating fluid is dispensed in the form of a continuous stream or flow.
The known coating methods in the continuous flow coating field include bead-coating and extrusion coating. In head-coating, a head of the coating material is established between a dispenser orifice and the surface to be coated and said bead is maintained by surface tension. Material is continuously withdrawn from the bead by the surface to be coated as it moves past the said dispenser orifice and the coating bead is continuously replenished from the coating material dispenser. By contrast, in extrusion coating, the coating material is forced through a coating applicator orifice, having a particular size and shape, and then onto the surface to be coated as said surface moves past the coating applicator in relatively close proximity thereto. The present invention applies to both such continuous flow coating methods.
In web coating processes wherein coating fluids flowing from a coating applicator are continuously deposited on a moving web spaced an optimum distance from a surface of the web (also referred to as the coating gap) variations in web coating thickness can result for any number of different reasons. Significant variations in web coating thickness can have an adverse effect on coated web properties. After extensive consideration and investigation, it has been determined that one of the many reasons for variations in web coating thickness is low-amplitude, low-frequency vibrations that are coupled to the coating fluid from various components of the coating apparatus. These investigations revealed that vibration of the coating fluid between the coating applicator and the adjacent moving web on which the coating fluid is deposited produces unwanted variations in web coating thickness.
Vibration of the coating fluid or bead (i.e., the coating fluid in the coating gap) can be kept within acceptable vibration levels by maintaining an extremely small coating gap such as an applicator-to-web coating gap of 2.0 mils. However, having a coating gap of this size produces additional web coating problems. One such problem is that of maintaining a uniform gap or applicator-to-web spacing across the entire width of the web to be coated, a web width that is normally in the vicinity of sixty inches. Gap variations across such a web may produce unacceptable variations in web coating thickness. Larger gaps of, for example, 5-10 mils would be significantly less difficult to maintain if such larger gaps could be employed. Another problem associated with small coating gaps is the necessity of periodically increasing gap size in order to enable relatively thick web-to-web splices to pass through a coating gap that would not otherwise permit such passage. This periodic gap size changing exercise raises web coating production costs in that web coating cannot be resumed until the proper gap spacing is re-established after the spliced web ends have passed through the temporarily enlarged coating gap. A further problem is the defects that are produced in a coating by small foreigh particles that will on occasion become lodged in a 2.0 mil coating gap.
A primary object of the present invention is to reduce vibration-induced variations in web coating thickness.
Another object of the present invention is to provide electromechanical vibration-generating apparatus whose vibrations can be coupled to the coating fluid in the coating gap of continuous flow coating apparatus and whose magnitude and frequency can be adjusted to reduce coating fluid vibrations.
A further object of the present invention is to provide continuous flow coating apparatus having a relatively large coating applicator-to-moving web coating gap.
Other objects, features and advantages of the present invention will be readily apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.